Process for graphitizing castings of white cast iron



Patented Apr. 21, 1931 UNITED STATES PATENT OFFICE ANSON HAYES, OF MIDDLETOWN, OHIO, ASSIGNOR, BY MESNE ASSIGNMENTS, TO IN- DUSTRIAL FURNACE CORPORATION, OF BUFFALO, NEW YORK, ACORPORATION OF NEW YORK P ROOESS FOR GRAPHITIZING GASTINGS OF WHITE CAST IRON No Drawing. Application filed July 29, 1927, Serial No. 209,389. Renewed September 11, 1930.

Y of the matters hereinafter described and more particularly pointed out in the appended claims.

The invention relates to certain novel improvements and developments in "the processes for the treatment of iron described in Letters Patent heretofore granted to me on the 23rd day of February, 1926, Nos. 1,574,374

and 1,574,375, wherein were described proc-' esses whereby the time theretofore required for annealing castings made from white cast iron was materially shortened.

The object of the present invention is to produce an improved method for bringing about complete graphitization'of the cast White iron; to still further reduce the time required for such graphitization; and to provide a method by which the time required for complete graphitization may be determined in the case of any white iron having a definite chemical composition.

These and other objects and advantages of the invention will appear more fully as I proceed with my specification.

In carrying out my improved process, I first subject the casting of white iron for a certain period of time to a constant temperature well above the critical temperature of the iron. That is to say, the temperature above which chemical changes would take place in the iron. This temperature is preferably about 1700 Fahrenheit but it may be above or below that point. The iron is then cooled to a point below the critical temperature, as for example and preferably to 1300 Fahrenheit at which temperature it is maintained for a second period of time. Maintaining the iron at this second temperature completes the graphitization of the pearlitic iron carbide after which'the iron is cooled to room temperature.

In my former patents I stated that the time required for annealing white cast iron depended not only upon the temperatures chosen but also upon the chemical composition of the white iron treated. By experiments I have been able to establish a definite relation between the chemical composition of the whlte cast iron and the annealing cycle required to bring about complete graphiti- .zation. These experiments have shown that the time required to bring about the graphitization of any fraction of the free iron carbide of such iron by holding it at a constant temperature above the critical range of the iron is a simple function of the composition of. the white iron in percentage of silicon, of phosphorous, of manganese and of sulphur, as well as of other factors hereinafter to be mentioned.

My experiments have further shown that the time at which. the iron shall be held at the temperature below the critical temperature in order to complete the graphitization of the pearlitic iron carbide is a simple function of its composition in percentage of carbon, of silicon, of phosphorous, of manganese and of sulphur, as well as of other factors which will hereinafter be mentioned.

In addition, I have found that if variations of the percentages of the several elements above named, which are normally present in white cast iron suitable for the manufacture of malleable iron, are held within sufficiently narrow limits, the time required to complete a desired degree of graph'itizat-ion either at the constant temperature above the critical range or at the constant temperature below the critical range, is a linear function of the percentage of any one of thesaid elements which influence the time at such temperature, if the percentages of the other said elements and also of the other factors which influence the time are held constant.

As one of the constants, a variation of which will be a determining factor in the time required, the specific formulae hereinafter given relate to irons which have been held during the refining period at a temperature of over 2800 Fahrenheit at a suiii ciently long time to obtain a condition of essential equilibrium throughout the molten charge; which have been poured into a mold for a temperature of about 2600 Fahrenheit; and which contain percentages of the aforesaid elements within the following linrits:carbon (C) from 1.9% to 2.85%; silicon (Si) from 1.35% to 0.7072); phosphorous (P) less than 0.25%; sulphur from 0.02% to 0.08%; and of manganese (Mn) of such percent as will give values of from 0.10 to 0.43 for the quantity percent of manganese minus twice the percent of sulphur (Mn2S).

I have determined by experiment that the time required to graphitize the free iron carbide at a temperature of 1700 Fahrenheit is given by the following equation:

(1) t 2-2.3(Si)12.4(P)+ 2.21(Mn2s) +331.

In this equation t is the time in hours and the symbols in the parentheses are, respectively, the percentages of silicon, of phosphorous, of manganese and of sulphur of the White cast iron under treatment.

For white cast iron of composition within the limits named, I have likewise found that the time required to complete the graphitization of the pearlitic iron carbide at a temperature of 1300 Fahrenheit is glven by the following equation:

2 t=23.6((l) -32.7(Si)+ io.9 ivin2s +38(P) +1027.

In this equation the symbols have the same meaning as in equation (1).

As will be manifest, these equations 1) and (2) may be written in the more general form wherein the symbols in the parentheses have the same meanings as in equations (1) and (2) and where K K K and K represent, respectively, the rate at which the time varies with the variation in the percentages of the carbon, silicon, phosphorous and of the percentage difference, Mn -2S, and where K is an integration constant. From this equation, it is manifest that each of equations (1) and (2) may be obtained by holding all except one of the elements constant, varying that one and calculating the result, then holding that element and all but one of the remaining elements constant, varying this one and calculating the result, and so on for each of the elements, thereby determining the coeiiicients for each of the elements in this way.

The application of equations like (3) may be made to calculate the time requiredto complete a desired degree of graphitizat on at a constant temperature and over wlde ranges of compositions of white iron. This may be done by using a sufliciently large number of equations, each of which is applied to a sufliciently limited range of compositions so that the coefficients may properly be considered as constants.

neomaa In carrying out the process, the percentages of the various elements hereinbefore men tioned, normally present in the white iron within the given limits, are first determined; the said percentages are then inserted in equations (1.) and (2) and said equations are solved to determine the required time for holding the said iron at the selected temperature above the critical range and the time re quired for holding the iron at the selected tem 'ierature below the critical range.

The process is then carried on as follows: The casting is heated to the selected tempcrature, namely 1700 Fahrenheit, in any suitable manner. It is then held at this temperature for the time determined by the solution of equation (1). The casting is then cooled slowly to the selected temperature be low the critical range, namely 1300 Fahrenheit. The cooling process is carried on at a rate which will not cause the formation of free cenientite (iron carbide) and may best be done by cooling the casting to a temperature somewhat above the critical range, as say 1500 Fahrenheit, and maintaining it at that temperature for a period of about six hours or less and then cooling it below the critical range to 1300 Fahrenheit. The casting is then held at this temperature for a period determined by a solution of equation (2). It is then cooled to room temperature.

The method herein described for determining the time required at 17 00 Fahrenheit to graphitize the free iron carbide and also the time necessary to complete the graphitization of the pearlitic iron carbide at 1300 F ahrenheit by combination, gives the time required for a complete annealing cycle.

In case it is desired or is necessary to use other temperatures than 1700 Fahrenheit for the high temperature treatment this may be done and the time at such temperature may be calculated by calculating the time required to complete the graphitization of the free iron carbide at 1700 Fahrenheit by using equation (1) and correcting this value as follows:

For temperatures between 1GO0 Fahrenheit and 1700 Fahrenheit add to the time required at 1700 Fahrenheit 15/100 of the time required at 1700 Fahrenheit for each 10 Fahrenheit that the temperature used is less than 1700 Fahrenheit. For temperatures between 1700 Fahrenheit and 1800 Fahrenheit subtract 6/100 of the time required at 1700 Fahrenheit for each 10 Fahrenheit that the temperature is higher than 1700 Fahrenheit.

In case it is desired or necessary to use temperatures higher or lower than 1300 Fahrenheit for graphitizing the pearlitic iron carbide this may also be done and the time required may be calculated in the following manner:

For temperatures between 1200 Fahrenheit and 1300 Fahrenheit add to the time required at 1300 Fahrenheit 93/1000 of the time required at 1300 Fahrenheit for each 10 Fahrenheit that the temperature used is lower than 1300 Fahrenheit. For tempera tures between 1300 Fahrenheit and 1400 Fahrenheit subtract from the time required at 1300 Fahrenheit 48/ 1000 of the time required at 1300 Fahrenheit for each 10 Fahrenheit that the temperature used is higher than 1300 Fahrenheit. In no case should a temperature above the lower critical temperature of the white iron be used.

Other annealing cycles using a greater number of stationary temperatures than are used in the annealing cycle already described and equations of the general type of (3) may be used to calculate the time most desirable to treat the iron at each such temperature without departing from my invention. An important feature of my invention lies in the selection of a suitable annealing cycle fora given composition of white cast iron and the control of such compositions so as to make feasible the use of a greatly reduced cycle of actual heating time.

The fact is recognized in this application that heating the molten iron to higher temperatures, the pouring of the molten iron into the. mold from higher temperatures, the thickness of section of the casting, higher temperatures of the mold at the time the casting is poured and in short any factors which tend to cause a coarse structure of the white iron casting, produce castings which take longer times for their graphitization. The times given here as necessary to complete the various stages of graphitization for any particular composition of white cast iron have been determined for such conditions which require the longest times for each such composition. The castings poured under the most favorable conditions for graphitization may graphitize in as much as 25 percent less time than that calculated.

Another advantage of my process and which I have discovered lies in the fact that many of the white iron compositions which are suitable for use in my improved process show no marked embrittling when cool-ed rapidly from temperatures between 700 Fahrenheit and 900 Fahrenheit. It is well known that the manufacturers of malleable iron have experienced very great difliculties with the production of white fracture malleable castings, when the malleabilized castlngs are reheated and cooled rapidly from the dlpp ng temperature, as in the hot dip galvanizing process or when they are cooled rapidly from temperatures in the vicinity of 850 Fahrenheit, after being cooled slowly from temperatures above 1100 Fahrenheit. This difiiculty is completely avoided by using phosphorous contents less than 0.12 percent for ranges of compositions of the other elements, within the values of the elements other than phosphorous, which are suitable for annealing cycles of about 60 hours actual time held at the selected temperatures. Another advantage of my process is that embrittling under the conditions of temperature recited in these specifications may be avoided in those white cast irons which contain from 0.12 to.0.25 percent phosphorous by coolingsuch castings rapidly from 1300 Fahrenheit in any suitable manner.

It will be manifest from the foregoing description that just as the time required for completely annealing a casting of white iron of given composition of the elements hereinbefore mentioned may be determined by the composition in percentages of carbon, silicon,

phosphorous, manganese and sulphur, which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for a certain period of time, and of thereafter subjecting the casting to a constant temperature below said critical temperature for a certain period of time, said periods of time being substantially such as would be measured by a predetermined constant in each case plus the algebraic sum of periods of time in proportion to the percentages of the said several elements of the composition.

2. The process for the graphitizing of castings from white cast iron of a predetermined composition in percentages of carbon, silicon,

temperature below said critical temperature for a certain period of time, said periods of time beingssubstantially such as would be measured by a predetermined constant in each case plus the algebraic sum of periods of time in proportion to the percentages of the said several elements of the composition.

3. The process for the graphitizing of castings from white cast iron of a predetermined composition in percentages of carbon, silicon,

phosphorous, manganese and sulphur (the said percentages of the several elements varying within predetermined narrow limits), which consists in subjecting the casting first to a constant temperature above the critical temperatures of the iron for a certain period of time, and of thereafter subjecting the casting to a constant temperature below said critical temperature for a certain period of time, said periods of time being substantially such as would be measured by a predetermined constant in each case plus the algebraic sum of periods of time in proportion to the percentages of the said several elements of the composition, and of thereafter cooling the casting to room temperature.

4. The procefs for the graphitizing of castings from white cast iron of a predetermined composition in percentages of carbon, silicon, phosphorous, manganese and sulphur (the said percentages of the several elements varying within predetermined narrow limits), which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for a certain period of time, in then cooling the casting to a point below the critical temperature carrying on the cooling process at a rate which will not cause the formation of free cementite (or iron carbide), and then subjecting the casting to a constant temperature below said critical temperature for a certain period of time, said periods of time being substantially such as would be measured by a predetermined constant in each case plus the algebraic sum of periods of time in proportion to the percentages of the said several elements of the composition, and of thereafter cooling the casting to room temperature.

5. The process for the graphitizing of castings from white cast iron, which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for a predetermined period of time, of then cooling the casting to a lower temperature somewhat above its critical temperature and maintaining it there for a predetermined period of time, in thereafter cooling the casting to a temperature below the critical temperature and maintaining it at that temperature for a predetermined length of time said predetermined times depending upon the com osition of the castings, and in then cooling t e casting to room temperature.

{5. The process for the graphitizing of castings from white cast iron of a predetermined composition in percentages of carbon, silicon, phosphorous, manganese and sulphur, which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for a certain period of time, of then cooling the casting to a lower temperature somewhat above the critical temperature of the iron and maintaining it at that temperature for a period of six hours or less, of then cooling the casting to a point below the critical temperature of the casting and holdin it at that temperature for a certain perio of time, the periods of time at which the casting is maintained at the first named higher temperature and at the last named lower temperature being substantially such as would be measured by a predetermined constant in each case plus the algebraic sum of periods of time in proportion to the percentages of the said several elements of the composition.

7. The process for the graphitizing of castings from white cast iron of a predetermined composition in percentages of carbon, silicon,

phosphorous, manganese and sulphur, which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for a certain period of time, or then cooling the casting to a lower temperature somewhat above the critical temperature of the iron and maintaining it at that temperature for a period of six hours or less, of then cooling the casting to a point below the critical temperature of the casting and holding it at that temperature for a certain period of time, the periods of time at which the casting is maintained at the first named higher temperature and at the last named lower temperature being substantially such .as would be measured by a predetermin ed constant in each case plus the algebraic sum of periods of time in proportion to the percentages of the said several elements of the composition, and of thereafter cooling the casting to room temperature.

8. The process for the graphitizing of castings from white cast iron of a predetermined composition in percentages of carbon, silicon, phosphorous, manganese and sulphur, which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for a certain period of time, of thereafter subjecting the casting to a constant temperature below said critical temperature for a certain period of time, said periods of time being substantially such as would be measured by a predeterminedconstant in each case plus the algebraic sum of periods of time in proportion to the percentages of the said several elements of the composition, and of then cooling said casting rapidly to room temperature.

9. The process for the graphitizing of castings from white cast iron of a predetermined composition in percentages of carbon, silicon, phosphorous, manganese and sulphur, which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for a certain period of time, in then cooling the casting to a point below the critical temperature carrying on the cooling process at a rate which will not cause the formation of free cementite (or iron carbide), of then subjecting the casting to a constant temperature below said critical temperature for a certain period of time, said periods of time being substantially such as would be measured by a predetermined constant in each case plus the algebraic sum of periods of time in proportion to the percentages of the said several elements of the composition, and of then cooling said casting rapidly to room temperature.

10. The process for the graphitizing of castings from white cast iron, which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for av predetermined period of time, in then cooling the casting to a point below the critical temperature carrying on the cooling process at a rate which will not cause the formation of free cementite, (or iron carbide), and in then subjecting the casting tosaid lower temperature for a .predeter mined period of time said predetermined times depending upon the composition of the castings.

11. The process for the graphitizing of castings from white cast iron of a predetermined composition in percentages of carbon, silicon. phosphorous, manganese and sulphur. which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for a certain period of time. and of thereafter subjectin the casting to a constant temperature below said critical temperature for a certain period of time, said periods of time being substantially such as would be measured by a predetermined constant in each case plus the algebraic sum of periods of time in proportion to the percentages of the said several elements of the composition, said" sum in cluding the carbon percentage for the second period andexcluding it for the first period.

12. The process for the graphitizing of castings from white cast iron of a predetermined composition in percentages of carbon, silicon, phosphorous, manganese and sulphur, which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for a certain period of time, in then cooling the casting to a point below the critical temperature carrying on the cooling process at a rate which will not cause the formation of tree ceinentite (or iron carbide), of then subjecting the casting to a constant temperature below said critical temperature for a certain period of time, said periods of time being substantially such as would be measured by a predetermined constant in each case plus the algebraic sum of periods of time in proportion to the percentages of the said several elements of the composition, said sum including the carbon percentage for the second period and excluding it for the first period.

13. The process for the graphitizing of castings from white cast iron of a predetermined composition in percentages of carbon,

silicon, phosphorous, manganese and sulphur (the said percentages of the several elements varying within predetermined narrow limits), which consists in subjecting the casting first to a constant temperature above the critical temperatures'of the iron for a certain period of time, and of thereafter subjecting the casting to a constant temperature below said critical temperature for a certain period of time, said periods of time being substantially such as would be measured by a predetermined constant in each case plus the algebraic sum of periods of time in proportion to the percentages of the said several elements of the composition, said sum including the carbon percentage for the second period and excluding it for the first period, and of thereafter cooling the castmg to room temperature.

14. The process for the graphitizing of castings from white cast iron of a predetermined composition 1n percentages of carbon, snicon, phosphorous, manganese and sul I phur (the said percentages of the several elements varyin g within predetermined narrow limits) which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for a certain period of time, in then cooling the casting to a point below the critical temperature carrying on the cooling process at a rate which will not cause the formation of free cementite (or iron carbide), of then subjecting the casting to a constant temperature below said critical temperature for a certain period of time, said periods of time being substantially such as would be meas ured by a predetermined constant in each case plus the algebraic sum of periods of time in proportion to the percentages of the said several elements of the composition, said sum including the carbon percentage for the second period and excluding it for the first period, and of thereafter cooling the casting to room temperature.

15. The process for the graphitizing of castings from white cast iron of a predetermined composition in percentages'of carbon, silicon, phosphorous, manganese and sulphur, which consists in subjecting the casting first to a constant temperature above the critical temperature oi the iron for a certain period of time, of then cooling the casting to a lower temperature somewhat above the critical temperature of the iron and maintaining it at that temperature for a period of six hours or less, of then cooling the casting to a point below the criticaltemperature of the casting and holding it at that temperature for a certain period of time, the periods of time at which the casting is maintained at the first named higher temperature and at the last named lower temperature being substantially such as would be measured by a predetermined constant in each case plus the algebraic sum of sum of periods of time in proportion to the periods of time in proportion to the percentages of the said several elements of the composition, said sum including the carbon percentage for the second period and excluding it for the first period.

16. The process for the graphitizing of castings from white cast iron of a predetermined composition in percentages of carbon, silicon, phosphorous, manganese and sulphur, which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for a certain period of time, of then cooling the casting to a lower temperature somewhat above the critical temperature of the iron and maintaining it at that temperature for a period of six hours or less, of then cooling the casting to a point below the critical temperature of the casting and holding it at that temperaure for a certain period of time, the periods of time at which the casting is maintained at the first named higher temperature and at the last named lower temperature being substantially such as would be measured by a predetermined constant in each case plus the algebraic percentages of the said several elements of the composition. said sum including the carbon percentage for the second period and excluding it for the first period, and of thereafter cooling the casting to room temperature.

17. The process for the graphitizing of castings from white cast iron of a predetermined composition in percentages of carbon,

silicon, phosphorous, manganese and sulphur,

which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for a certain period of time, of thereafter subjecting the casting to a constant temperature below said critical temperature for a certain period of time, said periods of time being substantially such as would be measured by a predetermined constant in each case plus the algebraic sum of periods of time in proportion to the percentages of the said several elements of the composition, said sum including the carbon percentage for the second period and excluding it for the first period, and of then cooling said casting rapidly to room temperature.

18. The process for the graphitizing of castings from white cast iron of a predetermined composition in percentages of carbon, silicon, phosphorous, manganese and sulphur, which consists in subjecting the casting first to a constant temperature above the critical temperature of the iron for a certain period of time, in then cooling the casting to a point below the critical temperature carrying on the cooling process at a rate which will not cause the formation of free cementite (or iron carbide), of then subjecting the casting to a constant temperature below said critical temperature for a certain period of time, said' 

